(1) Field of the Invention
The present invention relates to a magnetic disk device which reproduces information from a magnetic disk by using a magneto-resistive head wherein a sense current is supplied to the head and a change of a resistance of the head for the disk in a magnetic field is detected as a read data signal.
Recently, a magnetic disk device using a magneto-resistive (MR) head to reproduce information from a magnetic disk has been made widely available because the MR head provides a high output level of a read data signal when reading data from the magnetic disk. The MR head has a resistance which varies depending on a magnetic field applied to the magnetic disk. In the magnetic disk device, a sense current is supplied to the MR head as a bias current, and a change of a resistance of the MR head for the disk in the magnetic field is detected as a read data signal. In the magnetic disk device, the sense current supplied to the MR head is predetermined.
In a conventional magnetic disk device of the above type, the sense current supplied to the MR head is predetermined so that given reproduction characteristics of the MR head in a high-temperature operating condition are obtained and a useful life of the MR head in conformity with a given useful life of the magnetic disk device is satisfied.
(2) Description of the Related Art
In a conventional magnetic disk device using the MR head, the sense current supplied to the MR head is predetermined or fixed to a constant level so that the MR head provides given reproduction characteristics in a high-temperature operating condition, in order to allow the MR head to satisfy a useful life of the MR head in conformity with a given useful life of the device.
The sense current which is optimum to be supplied to the MR head varies depending on an operating condition of the MR head. Japanese Laid-Open Patent Application No. 8-293165 which is assigned to the applicant of the present invention discloses an automatic calibration method for a magnetic disk device having a plurality of MR heads. In the automatic calibration method of the above publication, a plurality of initialized sense current values with respect to the plurality of MR heads are predetermined according to the operating condition of each of the MR heads. In order to set the sense current at the optimum level, the sense current supplied to each of the MR heads is predetermined to be set to one of the initialized sense current values depending on the operating condition of each of the MR heads.
However, in the conventional magnetic disk device, the sense current supplied to the MR head is fixed to the initialized sense current value so that the reproduction characteristics of the MR head in the high-temperature operating condition are obtained and the useful life of the MR head in conformity with the desired life of the device is satisfied. Even when the MR head is operating at a room temperature or at low temperatures that are below the room temperature, the sense current supplied to the MR head is fixed to the initialized sense current value. In such a low-temperature operating condition, the sense current which is optimum to be supplied to the MR head is larger than the initialized sense current value. Hence, it is difficult for the conventional magnetic disk device to provide an adequate level of the reproduction characteristics when the MR head is in the low-temperature operating condition.
Further, the conventional magnetic disk device uses a magnetic head including the MR head, and an amount of levitation of the magnetic head from the surface of the magnetic disk becomes large when the magnetic head is operating at low temperatures. The level of the read data signal output by the MR head becomes low due to the increase of the amount of levitation in the low-temperature operating condition, and a read error rate of the conventional magnetic disk device tends to be increased when the MR head is in the low-temperature operating condition. It is difficult for the conventional magnetic disk device to provide an adequate level of the reproduction characteristics when the MR head is in the low-temperature operating condition.